The evolution of engagement and wedding ring styles moves at a glacial pace, but certain consumer preferences and a heightened awareness about gem quality are making a difference in today's market.

Simple rings are in. Elaborate is out, reports the Jewelers of America, a retail industry group. There is also a small but growing interest in antique engagement rings which can be something of a bargain and in the real bargains rings with simulated diamonds, the bigger the better.Other trends are toward bigger solitaire diamonds, cut in the marquise and emerald styles, yellow-gold settings and matching yellow-gold wedding bands.

"I would say the public wants to be more educated about buying diamonds than they used to be," said Peter Schneirla, senior gemologist and a vice president at Tiffany's in New York, the nation's No. 1 seller of engagement rings.

The brilliant-cut Tiffany round solitaire, in a six-pronged yellow-gold setting, was developed in the 1880s and remains the most popular style with American brides.

Although it sells more plain gold wedding bands for both brides and bridegrooms than any other style, Tiffany's reports a growing interest among women in wedding bands embellished with small round diamonds.

Jewelers of America reports that 91 percent of the nation's bridegrooms are wearing wedding bands. Tiffany's does not recommend or sell anything but plain gold bands for men but other popular styles include twisted or interlocking bands of yellow, white and pink gold, braided gold, or nail-head designs using two colors of gold. Brides often choose co-ordinated bands.

"Engaged couples are buying better quality rings," Schneirla reported. "In fact they are more interested in quality than size, although the size is going up. After all, a diamond ring is a major investment and should be a blue chip one."

The Gemology Institute of America rates diamonds according to whiteness and clearness on a scale starting with the letter "D" and descending to "Z."

"D," "E," and "F" rings are rare and extremely expensive. Most fine-quality engagement rings are of "G" or "H" quality.

The Diamond Information Center, a service of the international De Beers Diamond firm, reports the average weight of diamond solitaires set in engagment rings last year was .47 carats, up from .39 carats the previous year, with prices averaging around $1,l77.

The center also reported that of the 2.4 million brides in the United States in 1986, 1.8 million of them received engagement rings. Of that number, about 1.3 million were chosen by the couple.

On the simulated diamond scene, dealers indicated that many budget-minded couples will choose a fabulous fake and replace it with the real thing later.

"We have always done good business in engagement rings," said Mme. Wellington, the Washington, D.C.-based dealer whose simulated stones are considered among the best.

"Couples generally buy .50 or one-carat stones, the same size as the diamond they intend to buy later," she said. "The settings are 14 karat gold and can be used for the replacement."

A survey of simulated diamond shops showed that .50-carat stones sell for an average of $175 and one-carat stones for $250.

About 63 percent of brides-to-be prefer the brilliant round solitaire, but sales of marquise cut diamonds pointed at both ends doubled last year, and the square or rectangular emerald cut is becoming popular because of its "modern" look and its ability to make a diamond appear larger.

Antique rings are making their mark. A spokesman for Fred Leighton, a prominent dealer in antique and estate jewelry, said, "Antique rings tend to be unique and romantic and you don't have to pay the cost of today's labor in making the ring, because that's already been paid.

"A lot of people like the look of the types of faceting on old diamonds such as old European, old mine and rose cut," the spokesman said. "They represent a more personal expression of taste than the more modern and more common brilliant cut."

Gemstones have made some headway as engagement rings, thanks to the Princess Diana's sapphire-and-diamond number and the Duchess of York's ruby ring, but gemstones alone account for only 8 percent of ring sales.

"We never just sell a colored stone alone for an engagement ring," said Elizabeth Dolan, a spokeswoman for Cartier's. "They are always surrounded by small diamonds, a preference shown by the House of Windsor."